The present invention relates to the technical field of ankle prostheses for performing orthopedic treatment of the ankle joint by implementing prosthetic elements for restoring the anatomy of the ankle joint.
The present invention relates to an ankle prosthesis comprising a talus implant designed to be implanted in or on the talus or xe2x80x9cankle bonexe2x80x9d, and a top element including a tibia implant for implanting in or on the base of the tibia, said top element and the talus implant being mounted to move relative to each other with friction at a contact interface to allow movement of the ankle.
It is already known to use ankle prostheses after traumatic injury that has led to partial or total damage to the joint space of the ankle, in order to restore the joint anatomy of the ankle, at least in part.
Thus, ankle prostheses are already known comprising two implants, namely a talus implant for implanting in or on the talus, and a tibia implant for implanting in or on the base of the tibia. These two elements are implanted in the bones of the patient and they have congruent surfaces that are shaped in such a manner that the friction surface between the two implants is substantially spherical.
Hinge prostheses having two elements of the above-mentioned type confer satisfactory freedom of movement to an ankle joint restored in that way. Nevertheless, such prostheses make it possible to reproduce only some of the freedoms of movement of the natural ankle joint since the friction surface between the two implants is very different from the ideal physiological shape of the natural ankle joint. Since this is a constrained system, it leads to the patient""s capacity for movement being limited and gives rise to the element making up the prosthesis becoming detached at an increased rate.
That is why proposals have already been made to attempt to improve the freedom of ankle prosthesis movement by designing ankle prostheses having three elements, namely: a talus implant, a tibia implant, and an intermediate implant interposed between the tibia implant and the talus implant.
The intermediate implant is free to move between the other two implants and rests against the tibia implant. The contact area between the talus implant and the intermediate implant is generally constituted by a friction surface that is generally cylindrical or spherical in shape, as with two-element implants.
Consequently a three-element ankle prosthesis gives rise to greater freedom of movement for the ankle joint as restored in this way, thereby giving the patient the possibility of moving in a manner close to that allowed by the natural physiological shape of the ankle joint. Nevertheless, a three-element ankle prosthesis does not completely reproduce the natural physiological shape of the ankle joint. The natural operation of the ankle joint implies that the foot meets the ground laterally, followed by rolling of the foot which implies rotation that is directed progressively towards the inside of the foot. Consequently, the natural operation of the ankle is not restored by presently-known ankle prostheses.
Furthermore, presently-known ankle prostheses have locking means, e.g. studs, that are secured to the tibia implant so as to secure the implant to the tibia bone. Pressure cannot be applied while this implant is being implanted since the patient is prone. Thus, on first pressure against the ground, the tibia implant is pressed against the tibia bone giving rise to relative displacement between the tibia implant and the tibia bone. This displacement gives rise to high and uncontrolled levels of mechanical stress at the locking studs and the housings for receiving them, which can give rise to deformation or even to a risk of separation.
Consequently, the objects of the invention seek to propose a novel ankle prosthesis that does not present the drawbacks of the above-mentioned ankle prostheses and that reproduces as accurately as possible all of the displacement movements of the natural ankle joint.
Another object of the invention seeks to propose a novel ankle prosthesis that is particularly suited to reproducing the natural ankle joint by means of a three-implant prosthesis.
Another object of the invention seeks to propose a novel ankle prosthesis enabling good control to be obtained over the various relative movements of the implants constituting the prosthesis.
Another object of the invention seeks to propose an ankle prosthesis suitable for preventing or limiting the creation of any osteophyte impeding movement of the ankle.
Another object of the invention is to propose a novel ankle prosthesis suitable for improving the conditions under which the prosthesis is implanted and for preventing uncontrolled deformation of the implants after they have been implanted.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
The objects of the invention are achieved by means of an ankle prosthesis comprising a talus implant for implanting in or on the talus and a top element including a tibia implant for implanting in or on the base of the tibia, said top element and the talus implant being mounted to move relative to each other by friction on a contact interface so as to allow the ankle to move, wherein the contact interface presents a friction surface that can be considered as being a fraction of a substantially frustoconical surface, said surface being oriented so that its larger radius portion is directed substantially towards the outside of the ankle.